When sampling a waveform, in order to fully recover the original waveform it is necessary to use a sampling rate at least two times the highest waveform frequency. The Nyquist frequency, also called the Nyquist limit, is the highest frequency that can be coded at a given sampling rate in order to be able to fully reconstruct the original signal.
Software defined radios that use broadband digital receivers typically utilize high dynamic range, high speed analog-to-digital converters (ADCs) at the end of the analog receiver chain to digitize the received signal. ADCs are typically hardware devices and once implemented in the SDR, the ADC's capabilities are fixed.
The dynamic range (DR) of a radio receiver is essentially the range of signal levels over which the receiver can operate. The low end of the range is governed by the receiver's sensitivity while at the high end the receiver's sensitivity is governed by the receiver's overload or strong signal handling performance. Network-based wireless location systems must receive both strong locally generated radio signals and weaker signals generated in neighboring or geographically proximate cells to produce a Time-difference-of-arrival (TDOA) or TDOA/Angle of Arrival (AoA). Therefore, high DR radio receivers are high desirable in such systems. Furthermore, the need to deploy a plurality of geographically distributed receivers throughout the wireless network coverage area necessitates that the radio receivers be as inexpensive and robust as possible.
Increasingly, both uplink and downlink signals are used by the Uplink Time Difference of Arrival (U-TDOA) Wireless Location System (WLS) to locate mobile stations, base stations, and repeaters as disclosed in commonly assigned patent application Ser. No. 11/948,244 “Automated Configuration of a Wireless Location System” and Ser. No. 12/268,989 “Femto-cell Location by Direct Methods.”
High dynamic range receivers should have good performance at both ends of the receiver's dynamic range in order to detect both very low power levels and much higher power level signals. At the low end of the range, the receiver should have very high sensitivity by having a low noise figure so that the receiver can separate the signal from the noise. At the high end of the range the receiver should be sufficiently linear to receive higher level signals without distortion. Both of these performance goals may be limited by the bandwidth of the receiver. Therefore, the design of a high DR receiver that at the same time can operate over a broad bandwidth is a challenging task.
One such challenge is the near-far problem. The near-far (or hearability) problem may be defined as the problem of detecting and receiving a weaker signal in the presence of stronger signals. The near-far problem is a classic co-channel interference (also called cross-talk) problem typically arising in cellular frequency reuse networks. The near-far problem arises from the fact that radio signals from transmitters closer to the receiver of interest are received with smaller radio path-loss attenuation than with signals from transmitters located further away. Therefore, the strong signal from the nearby transmitter may mask the weak signal from the more distant transmitter. In Time Division Multiple Access (TDMA)/Frequency Division Multiple Access (FDMA) systems, channelization and frequency reuse patterns are used to limit the effect of the co-channel interference issue. In wideband (1.25 MHz and 5 MHz) direct sequence Code Division Multiple Access (CDMA) based wireless communications systems (IS-95, IS-2000, UMTS) the near-far effect, combined with imperfect orthogonality between codes, leads to substantial interference even with fast radio power control since multiple mobile devices may use the same channel. Since a network-based wireless location must be able to detect and in some cases demodulate signals arising from the local cell, neighboring cells and geographically proximate cells, the location system receivers must maintain a very stringent constraint on the dynamic linearity of the gain stage. Otherwise, harmonics or other spurious responses of the strong signal that are captured in the wrong frequency bin could easily hide the weaker signal-of-interest.
The sensitivity of the low noise performance of the receiver is affected by bandwidth (BW) as broadband components are typically higher in losses and the noise figure of Low Noise Amplifiers (LNAs) generally increases with increasing bandwidth. The linearity of the receiver is affected by increasing bandwidth in two ways. First, increasing instantaneous RF bandwidth allows more in-band signals that may create nonlinear products either by intermediation or by simple saturation due to their own power levels. Second, higher intermediate frequency (IF) bandwidths are more difficult to adequately filter before digitizing in the ADC. To maintain the dynamic range of the analog portion of the radio, the IF signal's images in adjacent Nyquist zones must be filtered before sampling in the ADC. The filter must reject adjacent images to at least the same level as the required dynamic range. As bandwidth increases, the required fractional bandwidth of the filter also increases making the filter more difficult to implement. The loss of the filter also increases and starts to affect dynamic range as well. For very high DR receivers, filter loss can be the limiting factor as the bandwidth is increased. This is especially true where Surface Acoustic Wave (SAW) filter technology is used to achieve very high dynamic range. SAW filter components also have very low insertion loss at high intermediate frequencies. Increasing the sampling rate may increase the width of the Nyquist zones and ease the rejection requirement of the IF filter because adjacent images are now farther away.
It would be advantageous if the above problems of designing a high DR receiver that at the same time can operate over a broad bandwidth can be addressed with a sampling technique to lower the cost, complexity, power consumption, and size of SDR receivers.